The present invention relates generally to the packaging of electronic components in electronic devices. More particularly, the present invention relates to the removable attachment of a heat sink to an integrated circuit package mounted on a circuit board in an electronic device.
As the art moves towards smaller higher power integrated circuits such as SRAMs, heat transfer from the integrated circuit package (IC package) becomes increasingly difficult and more important. As used herein, the term xe2x80x9cIC packagexe2x80x9d includes the heat generating integrated circuit as well as the packaging surrounding the integrated circuit.
One conventional technique to remove heat from an IC package is to employ a finned heat sink which is placed in thermal contact with the IC package. In this manner, heat generated by the IC package is conducted to the heat sink and then dissipated to the ambient environment.
Of importance, the heat sink must be reliably attached to the IC package in a manner which does not undesirably stress or damage the IC package or the circuit board to which the IC package is connected. One conventional technique is to employ a thermally conductive adhesive which bonds the heat sink to the IC package. However, thermally conductive adhesives do not adhere well to plastic IC packages resulting in an unacceptably high incidence of bond failure between the plastic IC package and the heat sink. Further, once the heat sink is bonded with the thermally conductive adhesive, it is difficult to remove the heat sink from the IC package without causing damage to the IC package, the heat sink or the circuit board. Yet, it is desirable to have a removable heat sink to readily allow chip repair, rework and/or replacement. Accordingly, it is desirable to avoid the use of thermally conductive adhesives altogether.
One prior art removable heat sink uses clips and/or fasteners to attach the heat sink directly to the IC package. However, when attached in this manner, the heat sink exerts undue force on the IC package which can damage and ultimately destroy the IC package. To avoid this problem, other removable heat sinks are attached directly to the circuit board to which the IC package is connected.
FIG. 1 is a side view of an electronic device 8 which includes a heat sink 10 directly attached by post type fastening members 12A to a circuit board 14 in accordance with the prior art. Located between heat sink 10 and circuit board 14 is an IC package 16 which generates heat during use. IC package 16 is typically electrically connected to circuit board 14 by one or more circuit interconnections, e.g., solder, which are not illustrated in FIG. 1 for purposes of clarity. Fastening members 12A urge heat sink 10 towards circuit board 14 and down onto IC package 16 to make the thermal contact between heat sink 10 and IC package 16.
Although providing the force necessary to make the thermal contact between heat sink 10 and IC package 16, fastening members 12A cause heat sink 10 to press unevenly on IC package 16. In particular, IC package 16 acts as a pivot between heat sink 10 and circuit board 14 so that end 10A of heat sink 10 is urged away from end 14A of circuit board 14 as indicated by arrows 18. This causes the force exerted by heat sink 10 on to IC package 16 to be greater at side 16A of IC package 16 than at side 16B. This uneven force distribution can damage and even crack IC package 16. Further, this uneven force distribution can create a gap between side 16B and heat sink 10 resulting in poor heat transfer between IC package 16 and heat sink 10. Alternatively, or in addition to, this uneven force distribution can cause circuit interconnection failure near side 16B of IC package 16. As those skilled in the art understand, these conditions can ultimately cause failure of device 8.
To avoid these drawbacks, it has become known in the art to attach both sides of heat sink 10 to circuit board 14. As an example, second post type fastening members 12B illustrated in dashed lines in FIG. 1 can be employed. This tends to equalize the force exerted by heat sink 10 on both sides 16A and 16B of IC package 16. However, this also causes the ends 14A, 14B of circuit board 14 to be pulled up by fastening members 12B, 12A, respectively, relative to the die attach region 14C of circuit board 14 to which IC package 16 is attached. This bending force, indicated by arrows 20, causes warpage of circuit board 14 such that circuit board 14 is displaced to a position 22. Over time, this warpage can cause device 8 to fail, e.g., from failure of circuit interconnections between IC package 16 and circuit board 14.
Accordingly, the art needs a heat sink assembly which results in even force application to the IC package and avoids circuit board warpage.
In accordance with the present invention, a heat sink assembly for an electronic device is presented. The assembly includes a retainer having a body with a first body edge and a second body edge, the second body edge being opposite and removed from the first body edge, a finger extending from the body along the first body edge and at least one leg extending from the body along the second body edge. The assembly further includes a heat sink having a base and fins extending from the base, where slits in the fins define a trench. In the finished assembly, an upper package which includes an electronic device is located between a substrate such as a circuit board and the heat sink, and the retainer holds the heat sink in place.
When attached to the heat sink, tension in the retainer causes the finger to apply a downward vertical force on the heat sink. The heat sink, in turn, presses downwards towards the upper package. Of importance, by locating a finger tip of the finger at the center of the heat sink, the heat sink presses down uniformly on the upper package.
In one embodiment, the upper package is attached to an upper surface of the circuit board, and a lower package which includes an electronic device is attached to a lower surface of the circuit board directly opposite the upper package. In accordance with this embodiment, the retainer includes two legs which are symmetric with respect to the finger, and each leg applies an equal upward force on the lower package. In this manner, a heat sink assembly in accordance with the present invention avoids the application of uneven force distribution on the upper and lower packages and the associated damage and ultimate device failure caused by use of conventional heat sink assemblies.
Further, the vertical upward force is transferred from the legs directly back to the heat sink through the lower package, the circuit board and the upper package without imparting any bending force on the circuit board. In this manner, a heat sink assembly in accordance with the present invention avoids circuit board warpage and the associated ultimate device failure associated with conventional heat sink assemblies.
In one embodiment, the body of the retainer is adjacent an edge of the circuit board so that the retainer passes around the circuit board instead of through the circuit board. The allows greater utilization of the circuit board, e.g., allows more traces and/or vias to be located on or in the circuit board, compared to the case where a retainer passes through the circuit board.
Also in accordance with the present invention, a method of assembling a heat sink assembly includes locating an upper package having an electronic device between a substrate such as a circuit board and a heat sink. A retainer is then positioned so that a finger of the retainer is aligned with a trench of the heat sink. The finger is then slid into the trench to secure the heat sink in place with the retainer.
While aligning the finger of the retainer with the trench of the heat sink, legs of the retainer are also aligned to slide around the circuit board. Thus, when the finger is slid into the trench, the legs also slide around the circuit board. Typically, the legs slide on a lower package attached to a lower surface of the circuit board, the lower package including an electronic device.
In some applications, installation of the retainer by the method described above may damage the assembly. For example, sliding of the legs along the lower package can damage the lower package or cause circuit interconnection failure between the lower package and the circuit board. To avoid potential damage to the assembly, the finger is spread apart from the legs. The retainer is then positioned without sliding of the retainer on the heat sink or on the lower package. The finger is then released thus securing the retainer to the heat sink.
In accordance with the present invention, a clip assembly tool for grasping the finger and legs of the retainer and spreading them apart from one another is presented. The tool includes a lower arm having a first clasp and an upper arm having a second clasp. The lower arm is pivotally attached to the upper arm. The tool further includes a wedge arm having a nose, where the wedge arm is pivotally attached to the upper arm.
A method of using the tool includes inserting the retainer between the first clasp and the second clasp, where the first and second clasps include first and second catches, respectively. The lower arm includes a first handle and the upper arm includes a second handle. The second handle is moved towards the first handle which causes the second clasp to move away from the first clasp allowing the retainer to be inserted between the first clasp and the second clasp. After the retainer is inserted, the second clasp is moved back towards the first clasp by moving the second handle away from the first handle.
The retainer is then wedged into the first catch and the second catch. To wedge the retainer, a third handle of the wedge arm is moved towards the second handle of the upper arm. This causes the nose of the wedge arm to press against the retainer and wedge the legs and finger of the retainer into the first and second catchs, respectively.
Once securely wedged, the retainer prevents the third handle of the wedge arm from being moved closer to the second handle of the upper arm. Thus, further motion of the third handle towards the first handle of the lower arm produces an equal downward motion of the second handle of the upper arm. As a result, the second clasp again moves away from the first clasp. Since the legs and the finger are securely fastened to the first and second catches, respectively, the finger is spread from the legs.
The finger is then inserted into the trench of the heat sink and the legs are positioned below the circuit board and the lower package attached to the lower surface of the circuit board. The tool is then operated by reversing the above described procedure to release the finger and to secure the retainer to the heat sink.
Of importance, the retainer is positioned and secured without sliding the finger along the heat sink and without sliding the legs along the lower package. Thus, use of the tool in accordance with the present invention avoids the possibility of damaging the heat sink assembly during installation of the retainer.